Not trying to pick a beef with Steve at all, very much respect his work. Or you for that matter. The question here is about workhorse PCs not gaming PCs. Gaming is a niche. Dell will absolutely sell you parts, or eBay or third party resellers. You've mentioned in your own post you can absolutely upgrade the GPU and PSU with standard consumer parts (even in a custom build you'd need cables specific to your modular power supply), even CPUs are upgradable within limits (again, with the exception of modern Ryzen when have you been able to upgrade more than 1 generation of CPU for a given socket?)
I'll 100% agree Dell don't conform to typical consumer standard parts that custom/small run builders use in a lot of cases, but to say they're not upgradable or repairable just isn't true. If someone was complaining their BMW gearbox wouldn't bolt onto their ford engine without some modification they'd struggle to find anyone who finds that a remotely strange situation.